Conventionally, there has been known an electric power steering apparatus which produces a steering assist torque so as to assist a driver's steering operation. Such an electric power steering apparatus detects a steering torque acting on a steering shaft by use of a torque detection device, calculates a target assist torque which increases with the steering torque, and feedback-controls the amount of electricity supplied to an electric motor so as to obtain the calculated target assist torque. Accordingly, in such an electric power steering apparatus, in particular, the torque detection device must be reliable.
A steering torque detection device detects the torsion angle of a torsion bar provided in a steering shaft, and calculates a steering torque which is proportional to the torsion angle. For example, a torque detection device disclosed in Patent Document 1 employs a configuration which detects the torsion angle of a torsion bar by making use of two resolvers. In this torque detection device, a first resolver is provided at one end of the torsion bar, and a second resolver is provided at the other end thereof. Steering torque is detected from the difference between the rotational angle (θ1) detected by the first resolver and the rotational angle (θ2) detected by the second resolver.
Each resolver includes an excitation coil to which an excitation AC signal is supplied and which energizes a rotor coil, and a sin phase detection coil and a cos phase detection coil fixedly disposed around the torsion bar. The sin phase detection coil and the cos phase detection coil are assembled with a shift of 90° (π/2) (electrical angle) provided therebetween. The sin phase detection coil outputs an AC signal having an amplitude corresponding to the sin value of the rotational angle of the rotor, and the cos phase detection coil outputs an AC signal having an amplitude corresponding to the cos value of the rotational angle of the rotor.
The two resolvers are connected to an ECU, which constitutes a torque computation section. The ECU supplies an excitation AC signal to the excitation coil of the first resolver and the excitation coil of the second resolver via a common excitation line. In addition, the ECU receives output signals from the respective detection coils of the first resolver and the second resolver via individual detection lines.
The ECU computes the rotational angles of the torsion bar at positions where the respective resolvers are provided from the output signals from the sin phase detection coil and the cos phase detection coil of each resolver. The ECU then detects the steering torque acting on the torsion bar from the difference between the two rotational angles.
In the torque detection device disclosed in Patent Document 1, even in the case where either one of two detection lines of either one of the two resolvers is broken, the ECU estimates the rotational angle by use of only the signal output from the normal detection line of the resolver which has the broken detection line so long as the rotational angle detected by the resolver which does not have the broken detection line is within a predetermined angle range. For example, in the case where a sin phase detection line (a detection line to which the sin phase detection coil is connected) of the first resolver is broken, the ECU estimates the rotational angle by use of only the output signal received via a cos phase detection line (a detection line to which the cos phase detection coil is connected) of the first resolver so long as the rotational angle detected by the second resolver is within the predetermined angle range. The ECU can make this estimation based on the assumption that the mechanical angular difference between the first rotational angle (θ1) detected by the first resolver and the second rotational angle (θ2) detected by the second resolver is always limited to a certain value or less. Accordingly, even in the case where either one of the two detection lines is broken, the ECU can detect steering torque from the difference between the first rotational angle θ1 and the second rotational angle θ2 so long as the rotational angle detected by the resolver whose detection lines are not broken is within the predetermined angle range.